Method of heat treating steel



Ap 1932- E; s. LAWRENCE 1,353,439

METHOD OF HEAT TREATING STEEL Filed April 22, 1929 2 Sheets-Sheet l L IH E Ill INVENTOR 0 WA PD 5. 1,4 WIPE/V65 ATTORN EY April 12, 1932.

,E. S. LAWRENCE METHOD OF HEAT TREATING STEEL Filed April 22, 1929 2Sheets-Sheet 2 INVENTOR EZJWABD 5 L4 WEE/V55 BY I ATTZRNEY Patented Apr.12, 1932 UNITED, STATES PATENT OFFICE EDWARD S. LAWRENCE, OF PITTSBURGH,PENNSYLVANIA, ASSIGNOR TO THE DURALOY COMPANY, OF PITTSBURGH,PENNSYLVANIA, A CORPORATION OF DELAWARE METHOD OF HEAT TREATING STEELApplication filed April 22, 19 29. I Serial No. 356,965.

This invention relates to the art of con tinuously heat treating sheetsteel, and more particularly to a method and apparatus for heat treatinglow tomedium carbon steel sheets or strips to give the steel deepdrawing properties. i I

Heretofore, the practice in producing drawing properties in low tomedium carbon steel sheets or strips, that is, steel containing carbonup to 35% has generally followed one or the other of two fairly welldefined heat treating processes. According to the first of such heattreating processes the hot rolled steel was first box annealed torelieve rolling strains. Then the boxes were removed from the furnaceand the steel when them into and withdraw them-from the furnaces.Moreover the physical properties and microstructure of the steel werenot such as would permit of the difficult drawing operations now beingdemanded in present manu facturing operations. According to the'sec- 0ndof such heat treating processes the -hot rolled steel was passed througha normalizing furnace to relieve rolling strains and caused arecrystallization of the grains of the steel, for example, a continuousfurnace of the type illustrated and described patents of A. T. KathnerNos. 1,725,398 and Reissue No. 17 ,413, the patent of Marsh & CochranNo. 1,610,567, or the patent of F. J. Winder No. 1,738,130. Thenormalized steel was pickled and in some cases cold rolled. Then it wasbox annealed to refine the pearlitic structure and to make the steelsuitable for diflicult drawing operations.

The normalizing step gave the steel basic properties desirable fordifficult drawing operations but these properties were brought out ordeveloped to the necessary extent only by the subsequent box annealingstep. This box annealing step had the same general effect on thenormalized steel, as regards softening the steel and refining thepearlitic structure, as it had on box annealed steel treated by thefirst described process. By this second process the normalizing furnacemade possible the elimination of the first box annealing step andconstituted a decided advance over that process in such respects as:

Length of time required for treatment of the steel; apparatus and spaceinvolved; labor costs in carrying out the method; and improvementin themicrostructure obtained in the steel.

By my present invention I am enabled to completely eliminate both of thebox annealing'steps heretofore used and yet to obtain physicalproperties and microstructural characteristics which render the treatedsteel especially thin gauge sheets susceptible to diflicult drawingoperations and which have not been obtained heretofore, so far as I amaware, except by processes which included the second box annealing step.

My improved method may be briefly described as consisting of the stepsof continuously heating and then coolin medium to low carbon steel insheet or strlp form. In the heating, the steel is heated to above theupper critical temperature range for such a length of time as isrequired to relieve substantially all strains set up in the steel byprevious operations and to permit a recrystallization of the steel.After the grains have grown to the desired size, the rowth of therecrystallized grains is arreste following which the steel is cooled insuch a manner that the recrystallized grains are converted into astructure in which the pearlite is changed from an emulsified orsorbitic state into a partially laminated condition. The

step of heating the steel to remove strains plished by passing therecrystallized struc-' ture into a cooler or cooling zone having a lowertemperature than the temperature at which the recrystallization iscarried out, this arresting or setting operation being controllable bymaintaining a predetermined ratio between the difference in temperaturesin the heating and cooling zones and the speed of travel of therecrystallized steel through the growth.

In the drawings forming a part of this specification Fig. 1 is a sideelevation, partly in section of afurnace embodying and suitable forpracticing my invention;

Figs. 2 and 3 are respectively cross sectional views through the heatingand cooling chambers of the furnace of Fig. 1;

Fig. 4 shows pearlite in an emulsified or sorbitic state at amagnification of about 750 diameters; and

Fig. 5 is a similar view showing the partially laminated pearlite.

In carryin out my method, I may employ a furnace of the generalconstruction disclosed in the above mentioned Kathner \Vinder and Marsh& Cochran patents, but have chosen to refer herein to a furnace of theKathner type. The herein described furnace is an elongated tunnel-likestructure provided with heating means 1 near one end thereof to heatthat ortion 2 of the furnace which may be' consi ere'd as a heatingzone.

The remainder of the furnace hasa lower roof and may be considered as acooling zone 3. I prefer to provide in the cooling zone 3 one or morebaflles 4, extending downwardly from the roof 5 to a point adjacent tothe top of the material passing through the furnace to deflectthe wastegases downwardly and about the cooling steel. The cooling zone 3 is muchlonger than the heating 'zone 2 and is sufliciently long to permit theformation of the partially laminated structure just described before thesteel emerges froni the furnace.

As illustrative of my invention, 18 gauge (U. S. S.) steel sheets orstrips having a carbon content of .1072; when treated according to myimproved method will have practically all hot rolling strains relievedwhile the steel is in the heatin zone of the furnace, and therecrystallized ferrite grains formed in the heating zone will be welldefined and thorcally no pearlite being visible. During the passage ofsuch a steel from the heating zone into the cooling zone the steel isreduced sufficiently in temperature so that no material grain growthtakes place. -The slow, gradual and uniform cooling of such steel in thecooling zone 3 of the furnace results in a change in phase condition ofthe pearlite from a sorbitic or emulsified state, as illustrated inFigure l, to a partially laminated state, as illustrated in Fig. 5. Thiscooling and change are important in imparting to the steel exceptionallygood drawing qualities. In Fig. at the pearlite is in an emulsifiedcondition and consists of particles 6 of ferrite and particlesli ofcementite- In Fig. 5 the cementite particles 7 have partially segregatedand are shown at 8 in a stratified conditionwhile the ferrite is shownat 9. Steel of the above mentioned carbon content and gauge when sotreated 1s in part characterlzed by a a hardness of from'about -10 to 48as measured on the B scale of a Rockwell hardness testing machine and inpart by properties which adapt it to difficult drawing operations. Sucha steel so treated has properties which are as good as or better thanthose of the same steel treated by processes which included the last boxannealing step.

The foregoing example of steel composition -is fairly typical ofcompositions of steel requiring ditlicult drawing properties but steelscontaining as much as .3570 of carbon are often SllbJQC'tQCl to stampingoperations and may be satisfactorily heat treated by my method andapparatus for such operations.

It will be understood that by my method clean steel is heatedsubstantially uniformly throughout all its parts and hence possesses asubstantially uniform recrystallized structure throughout. The operationof arresting the growth of the grains also takes place uniformlythroughout the steel and the slow and prolonged cooling is likewiseuniform throughout the steel so that the resultant product may be saidto be'uniform in all parts thereof.

As illustrative of the relative lengths of heating and cooling zones, Imay employ a furnace having a heating zone of about '75 feet in lengthand a cooling zone of about 125 feet or more in length when 18 gaugesteel sheets of 10% carbon content and 72 inches in width are to betreated singly or in packs in the amount of about 175 tons per day.

By my invention I am thus enabled to carry outin one continuousoperation the heat treatment of low to medium carbon strip or sheetsteel from the condition in which it comes from the hot rolling mill toa condition in which it possesses properties suitable for diflicultdrawing operations. I am able to so heat treat in about one hour aquantity of sheet or strip steel which by the previous processes wouldrequire about five days. My resultant product should be pickled andthereafter may be usedinnnediately. for various ture above the uppercritical temperature range thereof in a manner to relieve substan-'tially all rolling strains in the steel and to permit recrystallizationof the grains of the steel; arresting the growth of the recrystallizedgrains when they have attained a desired size; and then cooling thesteel substantially uniformly throughout all its portions in the coolingzone in a manner not only to prearrest grain growth and then cooled moreslowly until a partially laminated pearlitio structure is obtained. I

In testimony whereof I hereunto afiix my signature this 5th day ofApril, 1929.

EDWVARD S. LAWRENCE.

vent further substantial grain growth but i also to produce a partiallylaminated structure of pearlite, and thus heat treated steel having astructure and physical properties substantially equivalent to thestructure and physical properties of the same steel heat treated by aprocess which includes a final box annealing step.

2. The method of heat treating low to medium carbon sheet or strip steelwhich comprises passing the steel through communicating' heating andcooling zones, heating the steel in the heating zone to above the uppercritical temperature range of the steel, arresting the growth of thegrains when they have attained a desired size, and then cooling thesteel in a manner to produce a partially laminated pearlitic structurewhich is substantially uniform throughout all portions of the steel, thethus heat treated steel having a structure'and physical propertiessubstantially equivalent to the structure and physical properties of thesame steel heat treated by a process which includes a final boxannealing step.

3. The method of heat treating which comprises moving low to mediumcarbon sheet or strip steel progressively through an elongated zone ofsubstantially non-oxidizing gases, and, while the steelis in said zone,heating the steel to above its upper critical temperature range. thencooling the steel quickly and sutlicicntly to arrest grain growth andthen continuing the cooling at a slower rate until the pearlite of thesteel has attained a partially laminated structure.

4. The continuous method of heat treating of low to medium carbon sheetor strip steel which comprises progressively heating and cooling thesteel in the presence of substantiallly non-oxidizing gases, the steelbeing first heated to aboveits upper critical temperature range, thencooled quickly to

